U.S. patent number 7,615,774 [Application Number 11/118,575] was granted by the patent office on 2009-11-10 for aluminum free group iii-nitride based high electron mobility transistors.
This patent grant is currently assigned to Cree.Inc.. Invention is credited to Adam William Saxler.
United States Patent |
7,615,774 |
Saxler |
November 10, 2009 |
**Please see images for:
( Certificate of Correction ) ** |
Aluminum free group III-nitride based high electron mobility
transistors
Abstract
Aluminum free high electron mobility transistors (HEMTs) and
methods of fabricating aluminum free HEMTs are provided. In some
embodiments, the aluminum free HEMTs include an aluminum free Group
III-nitride barrier layer, an aluminum free Group III-nitride
channel layer on the barrier layer and an aluminum free Group
III-nitride cap layer on the channel layer.
Inventors: |
Saxler; Adam William (Durham,
NC) |
Assignee: |
Cree.Inc. (Durham, NC)
|
Family
ID: |
36540139 |
Appl.
No.: |
11/118,575 |
Filed: |
April 29, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20060244010 A1 |
Nov 2, 2006 |
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Current U.S.
Class: |
257/20; 257/194;
257/24; 257/E29.246 |
Current CPC
Class: |
H01L
29/7783 (20130101); H01L 29/2003 (20130101); H01L
29/205 (20130101) |
Current International
Class: |
H01L
29/06 (20060101); H01L 31/0328 (20060101); H01L
31/0336 (20060101); H01L 31/072 (20060101); H01L
31/109 (20060101) |
Field of
Search: |
;257/20,24,194,E29.246,E29.247,E29.248,E29.252,E29.253 |
References Cited
[Referenced By]
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|
Primary Examiner: Tran; Thien F
Attorney, Agent or Firm: Myers Bigel Sibley &
Sajovec
Claims
That which is claimed is:
1. A high electron mobility transistor (HEMT), comprising: an
aluminum free Group III-nitride barrier layer; an aluminum free
Group III-nitride channel layer on the barrier layer; an aluminum
free Group III-nitride cap layer on the channel layer; and a first
doped GaN layer between the barrier layer and the channel layer,
the first doped GaN layer comprising a Tin (Sn), Oxygen (O) and/or
Germanium (Ge) doped GaN layer.
2. The HEMT of claim 1, wherein the barrier layer comprises a doped
Group III-nitride region between the aluminum free Group
III-nitride channel layer and a substrate and wherein the barrier
layer is doped with iron (Fe).
3. The HEMT of claim 2, further comprising an undoped Group
III-nitride layer disposed between the doped Group III-nitride
region and the channel layer.
4. The HEMT of claim 1, wherein the barrier layer comprises a GaN
layer, the channel layer comprises an InGaN layer and the cap layer
comprises a GaN layer.
5. The HEMT of claim 4, wherein the barrier layer has a thickness
of from about 3.0 nm to about 1 mm, the channel layer has a
thickness of from about 0.3 nm to about 19 nm or from about 21 nm
to about 50 nm and the cap layer has a thickness of from about 1 nm
to about 19 nm or from about 21 nm to about 100 nm.
6. The HEMT of claim 4, wherein the InGaN layer has a percentage of
indium of from about 1% to about 9% or from about 11% to about
100%.
7. The HEMT of claim 1, wherein the first doped GaN layer has a
thickness of from about 0.2 nm to about 9 nm.
8. The HEMT of claim 1, wherein the first doped GaN layer has a
dopant concentration of from about 1.times.10.sup.17 cm.sup.-3 to
about 1.times.10.sup..about.cm.sup.-3 or from about
1.times.10.sup.20 cm.sup.-3 to about 1.times.10.sup.21
cm.sup.-3.
9. The HEMT of claim 1, further comprising a first undoped GaN
layer disposed between the first doped GaN layer and an InGaN
channel layer.
10. The HEMT of claim 9, wherein the first undoped GaN layer has a
thickness of from about 0.3 nm to about 4.5 nm or from about 5.5 nm
to about 10 nm.
11. The HEMT of claim 1, wherein the channel layer comprises a
doped Group III-nitride region on the aluminum free Group
III-nitride barrier layer.
12. The HEMT of claim 11, wherein the channel layer comprises an
InGaN channel layer and the barrier layer comprises a GaN barrier
layer, the InGaN channel layer comprising a doped region on the GaN
barrier layer.
13. The HEMT of claim 1, wherein the HEMT further comprises a
silicon carbide substrate and wherein the aluminum free Group
III-nitride barrier layer is on the silicon carbide substrate.
14. The HEMT of claim 1, wherein the barrier layer comprises an
undoped barrier layer.
Description
FIELD OF THE INVENTION
The present invention relates to semiconductor devices and, more
particularly, to transistors that incorporate nitride-based active
layers.
BACKGROUND
Materials such as silicon (Si) and gallium arsenide (GaAs) have
found wide application in semiconductor devices for lower power and
(in the case of Si) lower frequency applications. These, more
familiar, semiconductor materials may not be well suited for higher
power and/or high frequency applications, however, because of their
relatively small bandgaps (e.g., 1.12 eV for Si and 1.42 for GaAs
at room temperature) and/or relatively small breakdown
voltages.
In light of the difficulties presented by Si and GaAs, interest in
high power, high temperature and/or high frequency applications and
devices has turned to wide bandgap semiconductor materials such as
silicon carbide (2.996 eV for alpha SiC at room temperature) and
the Group III nitrides (e.g., 3.36 eV for GaN at room temperature).
These materials, typically, have higher electric field breakdown
strengths and higher electron saturation velocities as compared to
gallium arsenide and silicon.
A device of particular interest for high power and/or high
frequency applications is the High Electron Mobility Transistor
(HEMT), which, in certain cases, is also known as a modulation
doped field effect transistor (MODFET). These devices may offer
operational advantages under a number of circumstances because a
two-dimensional electron gas (2DEG) is formed at the heterojunction
of two semiconductor materials with different bandgap energies, and
where the smaller bandgap material has a higher electron affinity.
The 2DEG is an accumulation layer in the undoped ("unintentionally
doped"), smaller bandgap material and can contain a very high sheet
electron concentration in excess of, for example, 10.sup.13
carriers/cm.sup.2. Additionally, electrons that originate in the
wider-bandgap semiconductor transfer to the 2DEG, allowing a high
electron mobility due to reduced ionized impurity scattering.
This combination of high carrier concentration and high carrier
mobility can give the HEMT a very large transconductance and may
provide a strong performance advantage over metal-semiconductor
field effect transistors (MESFETs) for high-frequency
applications.
High electron mobility transistors fabricated in the gallium
nitride/aluminum gallium nitride (GaN/AlGaN) material system have
the potential to generate large amounts of RF power because of the
combination of material characteristics that includes the
aforementioned high breakdown fields, their wide bandgaps, large
conduction band offset, and/or high saturated electron drift
velocity. A major portion of the electrons in the 2DEG is
attributed to polarization in the AlGaN. HEMTs in the GaN/AlGaN
system have already been demonstrated. U.S. Pat. Nos. 5,192,987 and
5,296,395 describe AlGaN/GaN HEMT structures and methods of
manufacture. U.S. Pat. No. 6,316,793, to Sheppard et al., which is
commonly assigned and is incorporated herein by reference,
describes a HEMT device having a semi-insulating silicon carbide
substrate, an aluminum nitride buffer layer on the substrate, an
insulating gallium nitride layer on the buffer layer, an aluminum
gallium nitride barrier layer on the gallium nitride layer, and a
passivation layer on the aluminum gallium nitride active
structure.
Conventional HEMTs typically have an AlGaN layer on a GaN channel
layer. However, the presence of aluminum in the active region of
the device may reduce the reliability of the device as a result of
oxidation effects, dislocation related pits and/or the presence of
DX centers.
SUMMARY OF THE INVENTION
Some embodiments of the present invention provide high electron
mobility transistors (HEMTs) and methods of fabricating HEMTs that
include an aluminum free Group III-nitride barrier layer, an
aluminum free Group III-nitride channel layer on the barrier layer
and an aluminum free Group III-nitride cap layer on the channel
layer. In some embodiments of the present invention, the barrier
layer comprises a doped Group III-nitride region adjacent the
aluminum free Group III-nitride channel layer. An undoped Group
III-nitride layer may also be provided disposed between the doped
Group III-nitride region and the channel layer.
In additional embodiments of the present invention, the cap layer
comprises a first doped Group III-nitride region adjacent the
aluminum free Group III-nitride channel layer. An undoped Group
III-nitride layer may be disposed between the first doped Group
III-nitride region and the channel layer.
In some embodiments of the present invention, the barrier layer
comprises a GaN layer, the channel layer comprises an InGaN layer
and the cap layer comprises a GaN layer. The barrier layer may have
a thickness of from about 0.1 .mu.m to about 1000 .mu.m, the
channel layer may have a thickness of from about 1 nm to about 20
nm and the cap layer may have a thickness of from about 5 nm to
about 100 nm. The InGaN layer may have a percentage of indium of
from about 1 to about 100 percent.
In additional embodiments of the present invention, a first doped
GaN layer is disposed between the GaN barrier layer and the InGaN
channel layer. The first doped GaN layer may comprise a Si, Sn, O
and/or Ge doped GaN layer. The first doped GaN layer may have a
thickness of from about 0.2 nm to about 10 nm. The first doped GaN
layer may have a dopant concentration of from about
1.times.10.sup.16 cm.sup.-3 to about 1.times.10.sup.21 cm.sup.-3. A
first undoped GaN layer may be disposed between the first doped GaN
layer and the InGaN channel layer. The first undoped GaN layer may
have a thickness of from about 0.3 nm to about 5 nm.
In further embodiments of the present invention, a first doped GaN
layer is disposed between the GaN cap layer and the InGaN channel
layer. The first doped GaN layer disposed between the GaN cap layer
and the InGaN channel layer may comprise a Si, Sn, O and/or Ge
doped GaN layer. The first doped GaN layer disposed between the GaN
cap layer and the InGaN channel layer may have a thickness of from
about 0.2 nm to about 10 nm. The first doped GaN layer disposed
between the GaN cap layer and the InGaN channel layer may have a
dopant concentration of from about 1.times.10.sup.16 cm.sup.-3 to
about 1.times.10.sup.21 cm.sup.-3. A first undoped GaN layer may be
disposed between the first doped GaN layer and the InGaN channel
layer. The first undoped GaN layer disposed between the first doped
GaN layer and the InGaN layer may have a thickness of from about
0.3 nm to about 5 nm. A second doped GaN layer may be disposed
between the GaN barrier layer and the InGaN channel layer. The
second doped GaN layer disposed between the GaN barrier layer and
the InGaN channel layer may comprise a Si, Sn, O and/or Ge doped
GaN layer. The second doped GaN layer disposed between the GaN
barrier layer and the InGaN channel layer may have a thickness of
from about 0.2 nm to about 10 nm. The second doped GaN layer
disposed between the GaN barrier layer and the InGaN channel layer
may have a dopant concentration of from about 1.times.10.sup.16
cm.sup.-3 to about 1.times.10.sup.21 cm.sup.-3. A second undoped
GaN layer may be disposed between the second doped GaN layer and
the InGaN channel layer. The second undoped GaN layer may have a
thickness of from about 0.3 nm to about 5 nm.
In additional embodiments of the present invention, an InGaN layer
is provided on the GaN cap layer opposite the InGaN channel layer.
The InGaN layer on the GaN cap layer opposite the InGaN channel
layer may have a thickness of from about 0.3 nm to about 100
nm.
In some embodiments of the present invention, a metal semiconductor
field effect transistor (MESFET) is provided.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-section of an aluminum free Group III-nitride
based HEMT according to some embodiments of the present
invention.
FIG. 2 is a cross-section of an aluminum free GaN based HEMT
according to some embodiments of the present invention.
FIGS. 3A through 3E are cross-sections of aluminum free GaN based
HEMTs according to further embodiments of the present
invention.
FIGS. 4A through 4N are graphs of carrier concentration and band
diagrams from simulation models of transistors according to some
embodiments of the present invention.
FIG. 5 is a cross-section of an aluminum free GaN based HEMT
including an InGan layer on the cap layer according to some
embodiments of the present invention.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
The present invention now will be described more fully hereinafter
with reference to the accompanying drawings, in which embodiments
of the invention are shown. However, this invention should not be
construed as limited to the embodiments set forth herein. Rather,
these embodiments are provided so that this disclosure will be
thorough and complete, and will fully convey the scope of the
invention to those skilled in the art. In the drawings, the
thickness of layers and regions are exaggerated for clarity. Like
numbers refer to like elements throughout. As used herein the term
"and/or" includes any and all combinations of one or more of the
associated listed items.
The terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting of
the invention. As used herein, the singular forms "a", "an" and
"the" are intended to include the plural forms as well, unless the
context clearly indicates otherwise. It will be further understood
that the terms "comprises" and/or "comprising," when used in this
specification, specify the presence of stated features, integers,
steps, operations, elements, and/or components, but do not preclude
the presence or addition of one or more other features, integers,
steps, operations, elements, components, and/or groups thereof.
It will be understood that when an element such as a layer, region
or substrate is referred to as being "on" or extending "onto"
another element, it can be directly on or extend directly onto the
other element or intervening elements may also be present. In
contrast, when an element is referred to as being "directly on" or
extending "directly onto" another element, there are no intervening
elements present. It will also be understood that when an element
is referred to as being "connected" or "coupled" to another
element, it can be directly connected or coupled to the other
element or intervening elements may be present. In contrast, when
an element is referred to as being "directly connected" or
"directly coupled" to another element, there are no intervening
elements present. Like numbers refer to like elements throughout
the specification.
It will be understood that, although the terms first, second, etc.
may be used herein to describe various elements, components,
regions, layers and/or sections, these elements, components,
regions, layers and/or sections should not be limited by these
terms. These terms are only used to distinguish one element,
component, region, layer or section from another region, layer or
section. Thus, a first element, component, region, layer or section
discussed below could be termed a second element, component,
region, layer or section without departing from the teachings of
the present invention.
Furthermore, relative terms, such as "lower" or "bottom" and
"upper" or "top," may be used herein to describe one element's
relationship to another elements as illustrated in the Figures. It
will be understood that relative terms are intended to encompass
different orientations of the device in addition to the orientation
depicted in the Figures. For example, if the device in the Figures
is turned over, elements described as being on the "lower" side of
other elements would then be oriented on "upper" sides of the other
elements. The exemplary term "lower", therefore, encompasses both
an orientation of "lower" and "upper," depending of the particular
orientation of the figure. Similarly, if the device in one of the
figures is turned over, elements described as "below" or "beneath"
other elements would then be oriented "above" the other elements.
The exemplary terms "below" or "beneath" can, therefore, encompass
both an orientation of above and below.
Embodiments of the present invention are described herein with
reference to cross-section illustrations that are schematic
illustrations of idealized embodiments of the present invention. As
such, variations from the shapes of the illustrations as a result,
for example, of manufacturing techniques and/or tolerances, are to
be expected. Thus, embodiments of the present invention should not
be construed as limited to the particular shapes of regions
illustrated herein but are to include deviations in shapes that
result, for example, from manufacturing. For example, an etched
region illustrated as a rectangle will, typically, have tapered,
rounded or curved features. Thus, the regions illustrated in the
figures are schematic in nature and their shapes are not intended
to illustrate the precise shape of a region of a device and are not
intended to limit the scope of the present invention.
Unless otherwise defined, all terms (including technical and
scientific terms) used herein have the same meaning as commonly
understood by one of ordinary skill in the art to which this
invention belongs. It will be further understood that terms, such
as those defined in commonly used dictionaries, should be
interpreted as having a meaning that is consistent with their
meaning in the context of the relevant art and will not be
interpreted in an idealized or overly formal sense unless expressly
so defined herein.
It will also be appreciated by those of skill in the art that
references to a structure or feature that is disposed "adjacent"
another feature may have portions that overlap or underlie the
adjacent feature.
Embodiments of the present invention provide aluminum free
nitride-based HEMTs such as Group III-nitride based devices. As
used herein, the term "Group III nitride" refers to those
semiconducting compounds formed between nitrogen and the elements
in Group III of the periodic table, gallium (Ga), and/or indium
(In). As is well understood by those in this art, the Group III
elements can combine with nitrogen to form binary (e.g., GaN),
ternary (e.g., InGaN), and quaternary compounds. These compounds
all have empirical formulas in which one mole of nitrogen is
combined with a total of one mole of the Group III elements.
Accordingly, formulas such as In.sub.xGa.sub.1-xN where
0.ltoreq.x.ltoreq.1 are often used to describe them.
Furthermore, as used herein, aluminum free refers to the absence of
Al intentionally incorporated into active layers of the Group
III-nitride based device. For example, in some embodiments a region
or layer with less than about 1% Al may be considered aluminum
free. Thus, an active layer of a device may be Al free even if some
Al is present in the active layer, for example, through
unintentional incorporation by contamination during fabrication.
Active layers of a device are the layers of the device where the
2DEG is formed and layers disposed between the layers where the
2DEG is formed and the source, drain and/or gate contacts and/or
contact layers (i.e. layers on which a contact is directly formed)
of the device. Aluminum is not, however, intentionally incorporated
in the layers that form the 2DEG. Accordingly, in some embodiments
of the present invention, Al may be present in layers between the
layers that form the 2DEG and a substrate, in contacts and/or in
the substrate. For example, Al may be in the substrate, nucleation
and/or buffer layers and/or the ohmic contacts.
FIG. 1 illustrates a HEMT structure according to some embodiments
of the present invention. As seen in FIG. 1, a substrate 10 is
provided on which Group III-nitride based devices may be formed. In
particular embodiments of the present invention, the substrate 10
may be a silicon carbide (SiC) substrate that may be, for example,
4H polytype of silicon carbide. Other silicon carbide candidate
polytypes include the 3C, 6H, and 15R polytypes. In particular
embodiments of the present invention, the substrate 10 may be
semi-insulating. The term "semi-insulating" is used descriptively
rather than in an absolute sense. In particular embodiments of the
present invention, the silicon carbide bulk crystal has a
resistivity equal to or higher than about 1.times.10.sup.5
.OMEGA.-cm at room temperature. In other embodiments of the present
invention, the substrate 10 may be conductive.
Optional buffer, nucleation and/or transition layers (not shown)
may be provided on the substrate 10. For example, an AIN buffer
layer may be provided to provide an appropriate crystal structure
transition between the silicon carbide substrate and the remainder
of the device. Additionally, strain balancing transition layer(s)
may also be provided as described, for example, in commonly
assigned U.S. Patent Publication No. 2003/0102482A1, filed Jul. 19,
2002 and published Jun. 5, 2003, and entitled "STRAIN BALANCED
NITRIDE HETROJUNCTION TRANSISTORS AND METHODS OF FABRICATING STRAIN
BALANCED NITRIDE HETEROJUNCTION TRANSISTORS," and/or U.S. Pat. No.
6,841,001, entitled "STRAIN COMPENSATED SEMICONDUCTOR STRUCTURES
AND METHODS OF FABRICATING STRAIN COMPENSATED SEMICONDUCTOR
STRUCTURES," the disclosures of which are incorporated herein by
reference as if set forth fully herein.
Appropriate SiC substrates are manufactured by, for example, Cree,
Inc., of Durham, N.C., the assignee of the present invention, and
methods for producing are described, for example, in U.S. Pat. No.
Re. 34,861; U.S. Pat. Nos. 4,946,547; 5,200,022; and 6,218,680, the
contents of which are incorporated herein by reference in their
entirety. Similarly, techniques for epitaxial growth of Group III
nitrides have been described in, for example, U.S. Pat. Nos.
5,210,051; 5,393,993; 5,523,589; and 5,592,501, the contents of
which are also incorporated herein by reference in their
entirety.
Although silicon carbide may be used as a substrate material,
embodiments of the present invention may utilize any suitable
substrate, such as sapphire, aluminum nitride, aluminum gallium
nitride, gallium nitride, silicon, GaAs, LGO, ZnO, LAO, InP and the
like. In some embodiments, an appropriate buffer layer also may be
formed. For example, some embodiments of the present invention may
utilize thick semi-insulating or insulating Group III-nitride
layers and/or conducting substrates or layers as described in U.S.
patent application Ser. No. 11/103,127, filed Apr. 11, 2005 and
entitled "COMPOSITE SUBSTRATES OF CONDUCTIVE AND INSULTATING OR
SEMI-INSULATING GROUP III-NITRIDES FOR GROUP III-NITRIDE DEVICES,"
and/or U.S. patent application Ser. No. 11/103,117, filed Apr. 11,
2005 and entitled "THICK SEMI-INSULATING OR INSULATING EPITAXIAL
GALLIUM NITRIDE LAYERS AND DEVICES INCORPORATING SAME," the
disclosures of which are incorporated herein by reference as if set
forth in their entirety.
Returning to FIG. 1, an aluminum free Group III nitride barrier
layer 12 is provided on the substrate 10. An aluminum free Group
III-nitride channel layer 14 is provided on the barrier layer 12
and an aluminum free Group III-nitride cap layer 16 is provided on
the channel layer 14. The barrier layer 12 may be deposited on the
substrate 10 using buffer layers, transition layers, and/or
nucleation layers as described above. The barrier layer 12 may be
semi-insulating or insulating and/or may be unintentionally doped.
In some embodiments, the barrier layer 12 and/or the cap layer 16
may include doped regions adjacent the channel layer 14.
Furthermore, the barrier layer 12, channel layer 14, cap layer 16
and/or buffer nucleation and/or transition layers may be deposited
by MOCVD or by other techniques known to those of skill in the art,
such as MBE or HVPE.
The barrier layer 12 may be undoped or unintentionally doped. In
some embodiments, the barrier layer 12 may include a thick
semi-insulating or insulating layer with an undoped or
unintentionally doped region adjacent the channel layer 14. The
barrier layer 12 should be thick enough to prevent migration of Al
in layers disposed opposite the channel layer 14 to the channel
layer 14. Thus, portions of the barrier layer 12 may
unintentionally include Al while still being an aluminum free Group
III-nitride layer. For example, in some embodiments of the present
invention, the barrier layer 12 may be from about 1 nm to about
1.times.10.sup.6 nm thick and may have less than about 1% aluminum.
In some embodiments of the present invention, the barrier layer 12
is about 1000 .ANG. thick. Furthermore, a portion of the barrier
layer 12 distal from the channel layer 14 may be doped with Fe or
other elements to make it more insulating or provide a larger
barrier as described in the above referenced patent applications.
The barrier layer 12 may be provided as part or all of the
substrate 10 or as a separate layer on the substrate 10.
In some embodiments of the present invention, the channel layer 14
is a Group III-nitride, such as In.sub.xGa.sub.1-xN, where
0.ltoreq.x.ltoreq.1 provided that the energy of the conduction band
edge of the channel layer 14 is less than the energy of the
conduction band edge of the cap layer 16 at the interface between
the channel and cap layers. In particular, the channel layer 14 may
have a bandgap that is less than the bandgap of the cap layer 16
and the channel layer 14 may also have a larger electron affinity
than the cap layer 16. Embodiments of the present invention where
the channel layer 14 is in (i.e. x=1) may exhibit lower alloy
scatter because InN is a binary material. The channel layer 14 may
be undoped or unintentionally doped and may be grown to a thickness
of greater than about 10 .ANG.. For example, in some embodiments,
the channel layer 14 may have a thickness of from about 10 .ANG. to
about 200 .ANG.. The channel layer 14 may also be a multi-layer
structure, such as a superlattice or combinations of GaN, InGaN or
the like. In some embodiments of the present invention, the channel
layer 14 has less than about 1% aluminum.
In particular embodiments of the present invention, the cap layer
16 is thick enough and/or has a high enough doping to induce a
significant carrier concentration at the interface between the
channel layer 14 and the cap layer 16 through polarization effects.
As discussed above, the cap layer 16 may be a Group III-nitride and
has a bandgap larger than that of the channel layer 14 and a
smaller electron affinity than the channel layer 14. For example,
the cap layer 16 may be GaN or InGaN. If the cap layer 16 is InGaN
the cap layer 16 should have a lower indium percentage than is
present in the channel layer 14. The cap layer 16 may, for example,
be from about 5 nm to about 100 nm thick, but is not so thick as to
cause cracking or substantial defect formation therein. The cap
layer 16 may be thicker if the gate contact 24 is recessed into the
cap layer 16. In certain embodiments of the present invention, the
cap layer 16 is undoped and/or doped with an n-type dopant to a
concentration of 1.times.10.sup.16 cm.sup.-3 about
1.times.10.sup.21 cm.sup.-3. In some embodiments of the present
invention, the cap layer 16 has less than about 1% aluminum.
Source and drain ohmic contacts 20 and 22 are provided on the cap
layer 16 and a gate contact 24 is disposed between the source and
drain contacts 20 and 22. Suitable ohmic contact materials may
include, for example, Ti, Al, Ni and/or Au. Suitable gate materials
may depend on the composition of the cap layer, however, in certain
embodiments, conventional materials capable of making a Schottky
contact to a nitride based semiconductor material may be used, such
as Ni, Pt, NiSi.sub.x, Cu, Pd, Cr, W and/or WSiN.
FIG. 2 is a schematic diagram of HEMTs according to further
embodiments of the present invention. As seen in FIG. 2, a GaN
barrier layer 112 is provided on a substrate 110. The substrate 110
may be a substrate as described above with reference to the
substrate 10. Furthermore, optional buffer, nucleation and/or
transition layers (not shown) may be provided on the substrate 110
as described above. These optional buffer, nucleation and/or
transition layers may include aluminum. In particular embodiments
of the present invention, the substrate 110 is a GaN substrate.
As is further illustrated in FIG. 2, an InGaN channel layer 114 is
provided on the GaN barrier layer 112. A GaN cap layer 116 is
provided on the InGaN channel layer 114.
In particular embodiments of the present invention, the GaN barrier
layer 112 is a thick GaN layer and may be undoped, unintentionally
doped and/or semi-insulating or insulating. For example, the GaN
barrier layer 112 may be semi-insulating or insulating in a region
proximate the substrate 110 and may be undoped or unintentionally
doped in a region proximate the InGaN channel layer 114. The GaN
barrier layer 112 should be sufficiently thick to prevent migration
of Al in layers disposed opposite the channel layer 114 to the
channel layer 114. Thus, portions of the barrier layer 112 may
unintentionally include Al while still being an aluminum free
layer. In some embodiments of the present invention, the barrier
layer 112 may be from about 10 nm to about 1.times.10.sup.6 nm
thick. In particular embodiments of the present invention, the
barrier layer 112 is at least about 1000 .ANG. thick. In some
embodiments of the present invention, the barrier layer 112 has
less than about 1% aluminum.
The InGaN channel layer 114 may be In.sub.xG.sub.1-xaN, where
0<x.ltoreq.1 provided that the energy of the conduction band
edge of the channel layer 114 is less than the energy of the
conduction band edge of the cap layer 116 at the interface between
the channel and cap layers. In particular, the channel layer 114
may have a bandgap that is less than the bandgap of the cap layer
116 and the channel layer 114 may also have a larger electron
affinity than the cap layer 116. The channel layer 114 may be
undoped or unintentionally doped and may be grown to a thickness of
greater than about 10 .ANG.. For example, in some embodiments, the
channel layer 114 may have a thickness of from about 10 .ANG. to
about 200 .ANG.. The maximum thickness of the channel layer 114 may
depend on the percentage of indium in the channel layer 114. The
lower the percentage of indium in the channel layer 114, the
thicker the channel layer 114 may be before an undesirable two
dimensional hole gas is formed for Ga polar devices. A low or high
indium percentage may be desirable to reduce or minimize impurity
scattering. In particular embodiments of the present invention, the
indium percentage is the channel layer 114 is about 30% or less. In
some embodiments, the indium percentage is the channel layer 114 is
about 20%. In some embodiments of the present invention, the
channel layer 114 has less than about 1% aluminum.
The GaN cap layer 116 is thick enough and/or has a high enough
doping to induce a significant carrier concentration at the
interface between the channel layer 114 and the cap layer 116. In
some embodiments of the present invention, the GaN cap layer 116 is
from about 1 nm to about 100 nm thick, but is not so thick as to
cause cracking or substantial defect formation therein. In some
embodiments of the present invention, the cap layer 116 has less
than about 1% aluminum. As discussed above, with reference to the
cap layer 16, the cap layer 116 may be thicker if the gate contact
24 is recessed into the cap layer 116.
Optionally, an InGaN layer 117 may be provided on the GaN cap layer
116 as illustrated in FIG. 5 The InGaN layer 117 may increase the
barrier to the surface from the channel. If an InGaN 117 layer is
provided on the GaN cap layer 116, the InGaN layer 117 may have an
indium composition of from about 1% to 100% and may have a
thickness of from about 1 nm to about 100 nm.
FIGS. 3A through 3D are schematic illustrations of further
embodiments of HEMTs according the present invention having doped
and/or spacer layers adjacent an InGaN channel layer 214. As seen
in FIGS. 3A through 3D a GaN barrier layer 212 is provided on a
substrate 210. An InGaN channel layer 214 is provided on the GaN
barrier layer 212 and a GaN cap layer 216 is provided on the InGaN
channel layer 214. The substrate 210, GaN barrier layer 212, InGaN
channel layer 214 and GaN cap layer 216 may be provided as
described above with reference to the substrate 110, GaN barrier
layer 112, InGaN channel layer 114 and GaN cap layer 116 of FIG. 2.
The optional buffer, nucleation and/or transition layers described
above may also be provided. An optional InGaN layer (not shown) may
also be provided on the cap layer 216 as described above.
FIG. 3A illustrates embodiments of the present invention where a
doped GaN layer 230 is disposed between the GaN barrier layer 212
and the InGaN channel layer 214. In some embodiments of the present
invention, the doped GaN layer 230 may be doped with Si, Ge, Sn
and/or O and may have a dopant concentration of from about
1.times.10.sup.16 cm.sup.-3 to about 1.times.10.sup.21 cm.sup.-3.
In particular embodiments, the dopant concentration may be about
1.times.10.sup.20 cm.sup.-3. Furthermore, the doped GaN layer 230
may be from about 0.2 nm to about 10 nm thick. The doping
concentration should be high enough and the layer thick enough to
supply sufficient electrons to the 2DEG channel, but not so high or
thick as to have additional, unintentional n-type regions outside
of the channel region. In particular embodiments, the dopant may be
Sn and/or Ge. In other embodiments, the dopant may be Si. The doped
GaN layer 230 may be provided as a delta doped region. In
particular embodiments of the present invention, the doped layer
230 provides a sheet density of from about 1.times.10.sup.12
cm.sup.-2 to about 1.times.10.sup.14 cm.sup.-2 at the interface
with the channel layer 214.
While the doped layer 230 is described above with reference to a
GaN layer, in some embodiments of the present invention, the doped
layer 230 may be provided by an InGaN layer. Thus, for example, the
doped layer 230 may be provided by a doped region of the InGaN
channel layer 214. In such a case, the InGaN channel layer 214
should be thick enough and the doped portion thin enough and doped
lightly enough so that electrons from the doping are supplied to
the 2DEG and do not form an n-type region in the doped region.
FIG. 3B illustrates embodiments of the present invention where a
doped GaN layer 230 is disposed between the GaN barrier layer 212
and the InGaN channel layer 214 and an undoped GaN layer 240 is
disposed between the doped GaN layer 230 and the InGaN channel
layer 214. In some embodiments of the present invention, the
undoped GaN layer 240 may be from about 0.5 nm to about 5 nm thick.
The undoped GaN layer 240 may space the doped layer 230 from the
channel layer 214 to reduce and/or minimize impurity
scattering.
FIG. 3C illustrates embodiments of the present invention where a
doped GaN layer 250 is disposed between the GaN cap layer 216 and
the InGaN channel layer 214. In some embodiments of the present
invention, the doped GaN layer 250 may be doped with Si, Sn, Ge
and/or O and may have a dopant concentration of from about
1.times.10.sup.16 cm.sup.-3 to about 1.times.10.sup.21 cm.sup.-3.
Furthermore, the doped GaN layer 250 may be from about 0.2 nm to
about 100 nm thick. The structure of FIG. 3C could be used as a
MESFET with the InGaN channel layer 214 acting more as a back
barrier than a channel if the GaN layer 250 is doped heavily
enough. Mobility may be better in the doped GaN layer 250 than in
the InGaN channel layer 214 depending on the doping density and the
indium percentage.
FIG. 3D illustrates embodiments of the present invention where a
doped GaN layer 250 is disposed between the GaN cap layer 216 and
the InGaN channel layer 214 and an undoped GaN layer 260 is
disposed between the doped GaN layer 250 and the InGaN channel
layer 214. In some embodiments of the present invention, the
undoped GaN layer 260 may be from about 0.3 nm to about 10 nm
thick. The undoped GaN layer 260 may space the doped layer 250 from
the channel layer 214 to reduce and/or minimize impurity
scattering.
While embodiments of the present invention are illustrated in FIGS.
3A through 3D as including doped and/or undoped layers on one side
or the other of the InGaN channel layer 214, combinations and
subcombinations of the structures illustrated in FIGS. 3A through
3D may also be provided. For example, a structure with a doped
layer between the cap layer 216 and the channel layer 214 may also
have a doped layer between the barrier layer 212 and the channel
layer 214 as illustrated in FIG. 3E.
A passivation layer (not shown) may also be provided on the
structures of FIGS. 1 through 3D. In certain embodiments of the
present invention, the passivation layer may be silicon nitride,
aluminum nitride, silicon dioxide, an ONO structure and/or an
oxynitride. Furthermore, the passivation layer may be a single or
multiple layers of uniform and/or non-uniform composition.
FIGS. 4A through 4N are graphs of carrier concentration and band
diagrams from simulation models of transistors according to some
embodiments of the present invention. In the simulations depicted
in FIGS. 4A through 4N, the aluminum free layers are modeled as
having 0% aluminum. These simulations are not meant to be exact but
are provided to illustrate possible trends and to estimate
properties of different designs. Accordingly, these graphs are
provided as a rough estimate of possible characteristics of the
simulated device structures but are only as accurate as the
underlying assumptions and models. Accordingly, the properties of
actual devices may differ from those illustrated in FIGS. 4A
through 4N.
FIG. 4A illustrates a modeled band diagram and electron
concentration for an aluminum free HEMT with a thick undoped GaN
barrier layer, a 3 nm thick InGaN channel layer with 30% indium and
a 10 nm thick undoped GaN cap layer. FIG. 4B illustrates a modeled
band diagram and electron concentration for an aluminum free HEMT
with a thick undoped GaN barrier layer, a 3 nm thick InGaN channel
layer with 30% indium and a 20 nm thick undoped GaN cap layer. By
comparing FIGS. 4A and 4B, an increase in peak electron
concentration is predicted as a result of increasing the thickness
of the GaN cap layer.
FIG. 4C illustrates a modeled band diagram and electron
concentration for an aluminum free HEMT with a thick undoped GaN
barrier layer, a 1 nm thick doped GaN layer with a dopant
concentration of 18.times.10.sup.19 cm.sup.-3 between the barrier
layer and the channel layer, a 1 nm thick undoped GaN layer between
the doped GaN layer and the channel layer, a 6 nm thick InGaN
channel layer with 30% indium and a 60 nm thick undoped GaN cap
layer. As seen in FIG. 4C, the configuration of FIG. 4C is
predicted to have a higher peak electron concentration than either
of the configurations of FIGS. 4A and 4B.
FIG. 4D illustrates a modeled band diagram and electron
concentration for an aluminum free HEMT with a thick undoped GaN
barrier layer, a 1 nm thick doped GaN layer with a dopant
concentration of 20.times.10.sup.19 cm.sup.-3 between the barrier
layer and the channel layer, a 6 nm thick InGaN channel layer with
20% indium and a 60 nm thick undoped GaN cap layer. As seen in FIG.
4D, the configuration of FIG. 4D is predicted to have a higher peak
electron concentration than either of the configurations of FIGS.
4A and 4B but may have a lower peak electron concentration than
provided by the configuration of FIG. 4C.
FIG. 4E illustrates a modeled band diagram and electron
concentration for an aluminum free HEMT with a thick undoped GaN
barrier layer, a 1 nm thick doped GaN layer with a dopant
concentration of 10.times.10.sup.19 cm.sup.-3 between the barrier
layer and the channel layer, a 6 nm thick InGaN channel layer with
20% indium and a 60 nm thick undoped GaN cap layer. As seen in FIG.
4E, by increasing the dopant concentration in the doped GaN layer
and increasing the thickness of the GaN cap layer, the
configuration of FIG. 4E is predicted to have a higher peak
electron concentration than either of the configurations of FIGS.
4A and 4B but may have a lower peak electron concentration than
provided by the configuration of FIGS. 4C or 4D. The configuration
of FIG. 4E is predicted to have a higher conduction band edge in
the barrier layer than in FIG. 4D due to the lower doping.
FIG. 4F illustrates a modeled band diagram and electron
concentration for an aluminum free HEMT with a thick undoped GaN
barrier layer that is delta doped at 1.times.10.sup.13 cm.sup.-2 at
the interface with the channel layer, a 6 nm thick InGaN channel
layer with 20% indium and a 60 nm thick undoped GaN cap layer. As
seen in FIG. 4F, the configuration of FIG. 4F is about the same as
the configuration in FIG. 4E due to the same sheet doping density
in both structures with slightly lower conduction band bending in
the barrier due to the reduced thickness of the doped region.
FIG. 4G illustrates a modeled band diagram and electron
concentration for an aluminum free HEMT with a thick undoped GaN
barrier layer, a 3 nm thick doped GaN layer with a dopant
concentration of 3.times.10.sup.19 cm.sup.-3 between the barrier
layer and the channel layer, a 1 nm thick undoped GaN layer between
the doped GaN layer and the channel layer, a 3 nm thick InGaN
channel layer with 30% indium and a 20 nm thick undoped GaN cap
layer. As seen in FIG. 4G, the configuration of FIG. 4G is
predicted to have a higher peak electron concentration than either
of the configurations of FIGS. 4A and 4B.
FIG. 4H illustrates a modeled band diagram and electron
concentration for an aluminum free HEMT with a thick undoped GaN
barrier layer, a 3 nm thick doped GaN layer with a dopant
concentration of 3.times.10.sup.19 cm.sup.-3 between the barrier
layer and the channel layer, a 1 nm thick undoped GaN layer between
the doped GaN layer and the channel layer, a 3 nm thick InGaN
channel layer with 30% indium and a 30 nm thick undoped GaN cap
layer. As seen in FIG. 4H, the configuration of FIG. 4H is
predicted to have a slightly higher peak electron concentration
than the configuration of FIG. 4G as a result of the thicker GaN
cap.
FIG. 4I illustrates a modeled band diagram and electron
concentration for an aluminum free HEMT with a thick undoped GaN
barrier layer, a 3 nm thick doped GaN layer with a dopant
concentration of 3.times.10.sup.19 cm.sup.-3 between the barrier
layer and the channel layer, a 1 nm thick undoped GaN layer between
the doped GaN layer and the channel layer, a 3 nm thick InGaN
channel layer with 20% indium and a 30 nm thick undoped GaN cap
layer. As seen in FIG. 4I, the configuration of FIG. 4I is
predicted to have a lower peak electron concentration than the
configurations of FIG. 4H due to the lower In percentage.
FIG. 4J illustrates a modeled band diagram and electron
concentration for an aluminum free HEMT with a thick undoped GaN
barrier layer, a 3 nm thick doped GaN layer with a dopant
concentration of 3.times.10.sup.19 cm.sup.-3 between the cap layer
and the channel layer, a 1 nm thick undoped GaN layer between the
doped GaN layer and the channel layer, a 3 nm thick InGaN channel
layer with 20% indium and a 26 nm thick undoped GaN cap layer. As
seen in FIG. 4J, the configuration of FIG. 4J is predicted to have
two peaks in the electron concentration and has a lower peak
electron concentration than configurations with a doped layer on
the opposite side of the channel layer as seen in FIG. 4I. A
structure similar to that of FIG. 4J could be used as a MESFET, as
mentioned above.
FIG. 4K illustrates a modeled band diagram and electron
concentration for an aluminum free HEMT with a thick undoped GaN
barrier layer, a 3 nm thick doped GaN layer with a dopant
concentration of 3.times.10.sup.19 cm.sup.-3 between the barrier
layer and the channel layer, a 1 nm thick undoped GaN layer between
the doped GaN layer and the channel layer, a 6 nm thick InGaN
channel layer with 20% indium and a 30 nm thick undoped GaN cap
layer. In comparison to the structure of FIG. 4I, the structure of
FIG. 4K has a higher back barrier due to the thicker InGaN layer.
See U.S. patent application Ser. No. 10/772,882, filed Feb. 5,
entitled "NITRIDE HETEROJUNCTION TRANSISTORS HAVING CHARGE-TRANSFER
INDUCED ENERGY BARRIERS AND METHODS OF FABRICATING THE SAME," the
disclosure of which is incorporated herein as if set forth fully
herein.
FIG. 4L illustrates a modeled band diagram and electron
concentration for an aluminum free HEMT with a thick undoped GaN
barrier layer, a 3 nm thick doped GaN layer with a dopant
concentration of 3.times.10.sup.19 cm.sup.-3 between the barrier
layer and the channel layer, a 1 nm thick undoped GaN layer between
the doped GaN layer and the channel layer, a 6 nm thick InGaN
channel layer with 20% indium and a 60 nm thick undoped GaN cap
layer. As seen in FIGS. 4K and 4L, a thicker GaN cap may increase
the charge in the channel.
FIGS. 4M and 4N illustrate modeling of configurations that vary the
thickness of the GaN doped layer. The structure of FIG. 4M has a
higher In concentration than the structure of FIG. 4L, resulting in
a higher carrier concentration and higher conduction band in the
barrier, but a lower mobility due to increased alloy scattering is
likely. As seen in FIGS. 4M and 4N, FIG. 4N has a thicker doped
layer than FIG. 4M and, therefore, a higher electron concentration
in the channel and a lower conduction band energy in the barrier
layer.
An aluminum free HEMT structure according to some embodiments of
the present invention has been fabricated using a 60 nm GaN cap
layer, a 6 nm InGaN channel layer with 20% In and a
1.7.times.10.sup.13 cm.sup.-2 Si delta doped region at the
interface with a thick GaN barrier layer. Such device structure
exhibited a sheet resistivity of approximately 1200
.OMEGA./.quadrature..
While embodiments of the present invention have been described
herein with reference to particular HEMT structures, the present
invention should not be construed as limited to such structures.
For example, additional layers may be included in the HEMT device
while still benefiting from the teachings of the present invention.
In some embodiments, insulating layers such as SiN, an ONO
structure or relatively high quality AlN may be deposited for
making a MISHEMT and/or passivating the surface. The additional
layers may also include a compositionally graded transition layer
or layers.
Also, other structures, such as recessed or "T" gate structures,
regrown contact regions or the like may also be provided.
Accordingly, some embodiments of the present invention provide
aluminum free embodiments of structures such as those described in,
for example, U.S. Pat. No. 6,316,793 and U.S. Patent Publication
No. 2002/0066908A1 filed Jul. 12, 2001 and published Jun. 6, 2002,
for "ALUMINUM GALLIUM NITRIDE/GALLIUM NITRIDE HIGH ELECTRON
MOBILITY TRANSISTORS HAVING A GATE CONTACT ON A GALLIUM NITRIDE
BASED CAP SEGMENT AND METHODS OF FABRICATING SAME," U.S. Pat. No.
6,849,882 to Smorchkova et al., entitled "GROUP-III NITRIDE BASED
HIGH ELECTRON MOBILITY TRANSISTOR (HEMT) WITH BARRIER/SPACER
LAYER", U.S. patent application Ser. No. 10/617,843 filed Jul. 11,
2003 for "NITRIDE-BASED TRANSISTORS AND METHODS OF FABRICATION
THEREOF USING NON-ETCHED CONTACT RECESSES," U.S. patent application
Ser. No. 10/772,882 filed Feb. 5, 2004 for "NITRIDE HETEROJUNCTION
TRANSISTORS HAVING CHARGE-TRANSFER INDUCED ENERGY BARRIERS AND
METHODS OF FABRICATING THE SAME," U.S. patent application Ser. No.
10/897,726, filed Jul. 23, 2004 entitled "METHODS OF FABRICATING
NITRIDE-BASED TRANSISTORS WITH A CAP LAYER AND A RECESSED GATE,"
U.S. patent application Ser. No. 10/849,617, filed May 20, 2004
entitled "METHODS OF FABRICATING NITRIDE-BASED TRANSISTORS HAVING
REGROWN OHMIC CONTACT REGIONS AND NITRIDE-BASED TRANSISTORS HAVING
REGROWN OHMIC CONTACT REGIONS," U.S. patent application Ser. No.
10/849,589, filed May 20, 2004 and entitled "SEMICONDUCTOR DEVICES
HAVING A HYBRID CHANNEL LAYER, CURRENT APERTURE TRANSISTORS AND
METHODS OF FABRICATING SAME," U.S. Patent Publication No.
2003/0020092 filed Jul. 23, 2002 and published Jan. 30, 2003 for
"INSULATING GATE ALGAN/GAN HEMT", U.S. patent application Ser. No.
10/996,249, filed Nov. 23, 2004 and entitled "CAP LAYERS AND/OR
PASSIVATION LAYERS FOR NITRIDE-BASED TRANSISTORS, TRANSISTOR
STRUCTURES AND METHODS OF FABRICATING SAME," U.S. patent
application Ser. No. 11/080,905 filed Mar. 15, 2005 and entitled
"GROUP III NITRIDE FIELD EFFECT TRANSISTORS (FETs) CAPABLE OF
WITHSTANDING HIGH TEMPERATURE REVERSE BIAS TEST CONDITIONS," U.S.
patent application Ser. No. 11/005,107, filed Dec. 6, 2004 and
entitled "HIGH POWER DENSITY AND/OR LINEARITY TRANSISTORS," and
U.S. patent application Ser. No. 11/005,423, filed Dec. 6, 2004 and
entitled "FIELD EFFECT TRANSISTORS (FETs) HAVING MULTI-WATT OUTPUT
POWER AT MILLIMETER-WAVE FREQUENCIES," the disclosures of which are
incorporated herein as if described in their entirety. Embodiments
of the present invention may also be utilized with HEMT structures
such as described in, for example, Yu et al., "Schottky barrier
engineering in III-V nitrides via the piezoelectric effect,"
Applied Physics Letters, Vol. 73, No. 13, 1998, or in U.S. Pat. No.
6,584,333 filed Jul. 12, 2001, for "ALUMINUM GALLIUM
NITRIDE/GALLIUM NITRIDE HIGH ELECTRON MOBILITY TRANSISTORS HAVING A
GATE CONTACT ON A GALLIUM NITRIDE BASED CAP SEGMENT AND METHODS OF
FABRICATING SAME," the disclosures of which are incorporated herein
by reference as if set forth fully herein.
In the drawings and specification, there have been disclosed
typical embodiments of the invention, and, although specific terms
have been employed, they have been used in a generic and
descriptive sense only and not for purposes of limitation, the
scope of the invention being set forth in the following claims.
* * * * *
References